Mohini Sain

CELLULOSIC NANOCOMPOSITES: A REVIEW

Because of their wide abundance, their renewable and environmentally benign nature, and their outstanding mechanical properties, a great deal of attention has been paid recently to cellulosic nanofibrillar structures as components in nanocomposites. A first major challenge has been to find efficient ways to liberate cellulosic fibrils from different source materials, including wood, agricultural residues, or bacterial cellulose. A second major challenge has involved the lack of compatibility of cellulosic surfaces with a variety of plastic materials. The water-swellable nature of cellulose, especially in its non-crystalline regions, also can be a concern in various composite materials. This review of recent work shows that considerable progress has been achieved in addressing these issues and that there is potential to use cellulosic nano-components in a wide range of high-tech applications.

FLAME RETARDANT AND MECHANICAL PROPERTIES OF NATURAL FIBRE-PP COMPOSITES CONTAINING MAGNESIUM HYDROXIDE

Flammability of polypropylene, sawdust/rice husk filled polypropylene composites and flame retarding effect of magnesium hydroxide for these composites was studied by horizontal burning rate and oxygen index tests. Effect of flame-retardants such as boric acid or zinc borate in combination with magnesium hydroxide was also studied. It was found that magnesium hydroxide can effectively reduce the flammability (almost 50%) of natural fibre filled polypropylene composites. No synergetic effect was observed when magnesium hydroxide was used in combination with boric acid and zinc borate. Marginal reduction in the mechanical properties of the composites was found with addition of flame-retardants.

Processing of Cellulose Nanofiber-reinforced Composites

Cellulose nanofibers are obtained from various sources such as flax bast fibers, hemp fibers, kraft pulp, and rutabaga, by chemical treatments followed by innovative mechanical techniques. The nanofibers thus obtained have diameters between 5 and 60 nm. The ultrastructure of cellulose nanofibers is investigated by atomic force microscopy and transmission electron microscopy. The cellulose nanofibers are also characterized in terms of crystallinity. Reinforced composite films comprising 90% polyvinyl alcohol and 10% nanofibers are also prepared. The comparison of the mechanical properties of these composites with those of pure PVA confirmed the superiority of the former.

Cellulose microfibrils: A novel method of preparation using high shear refining and cryocrushing

Abstract This paper describes a novel technique to produce cellulose microfibrils through mechanical methods. The technique involved a combination of severe shearing in a refiner, followed by high-impact crushing under liquid nitrogen. Fibers treated in this way were subsequently either freeze-dried or suspended in water. The fibers were characterized using SEM, TEM, AFM, and high-resolution optical microscopy. In the freeze-dried batch, 75% of the fibrils had diameters of 1 μm and below, whereas in the water dispersed batch, 89% of the fibrils had diameters in this range. The aspect ratio of the microfibrils ranged between 15 and 55 for the freeze-dried fibrils, and from 20 to 85 for the fibrils dispersed in water. These measurements suggest that the microfibrils have the potential to produce composites with high strength and stiffness for high-performance applications. The microfibrils in water were compounded with polylactic acid polymer to form a biocomposite. Laser confocal microscopy showed that the microfibrils were well dispersed in the polymer matrix.

Carbon storage potential in natural fiber composites

Abstract The environmental performance of hemp based natural fiber mat thermoplastic (NMT) has been evaluated in this study by quantifying carbon storage potential and CO2 emissions and comparing the results with commercially available glass fiber composites. Non-woven mats of hemp fiber and polypropylene matrix were used to make NMT samples by film-stacking method without using any binder aid. The results showed that hemp based NMT have compatible or even better strength properties as compared to conventional flax based thermoplastics. A value of 63 MPa for flexural strength is achieved at 64% fiber content by weight. Similarly, impact energy values (84–154 J/m) are also promising. The carbon sequestration and storage by hemp crop through photosynthesis is estimated by quantifying dry biomass of fibers based on one metric ton of NMT. A value of 325 kg carbon per metric ton of hemp based composite is estimated which can be stored by the product during its useful life. An extra 22% carbon storage can be achieved by increasing the compression ratio by 13% while maintaining same flexural strength. Further, net carbon sequestration by industrial hemp crop is estimated as 0.67 ton/h/year, which is compatible to all USA urban trees and very close to naturally, regenerated forests. A comparative life cycle analysis focused on non-renewable energy consumption of natural and glass fiber composites shows that a net saving of 50 000 MJ (∼3 ton CO2 emissions) per ton of thermoplastic can be achieved by replacing 30% glass fiber reinforcement with 65% hemp fiber. It is further estimated that 3.07 million ton CO2 emissions (4.3% of total USA industrial emissions) and 1.19 million m3 crude oil (1.0% of total Canadian oil consumption) can be saved by substituting 50% fiber glass plastics with natural fiber composites in North American auto applications. However, to compete with glass fiber effectively, further research is needed to improve natural fiber processing, interfacial bonding and control moisture sensitivity in longer run.

ISOLATION OF CELLULOSE MICROFIBRILS – AN ENZYMATIC APPROACH

Isolation and application of cellulose nano-fibers and microfibrils are expanding rapidly due to their environmental benefits and specific strength properties, especially in bio-composite science. In this research work, we have successfully developed and explored a novel bio-pretreatment for wood fibre that can substantially improve the yield of the current mechanical process applied to isolate cellulose microfibrils. Microfibrils are isolated in the laboratory through a combination of high shear refining, and subsequent cryocrushing under the presence of liquid nitrogen but at the expense of high-energy and is hampering the current momentum in the direction of their isolation on a sizable scale for any potential application. Any attempt to loosen up the microfibrils by either complete or partial destruction of the hydrogen bonds before the mechanical process will be a step forward in the quest for economical isolation of cellulose microfibrils. Bleached Kraft spruce Pulp was treated with OS1, a fungus isolated from Dutch Elm tree infected with Dutch elm disease at different treatment conditions. Cellulose microfibrils were isolated from these treated fibers by high shear refining followed by cryocrushing. The % yield of cellulose microfibrils based on their diameter showed a significant shift towards a lower diameter range after the high shear refining compared to yield of cellulose microfibrils from untreated fibres. The overall yield of cellulose microfibrils from the treated fibres did not show any sizeable decrease.

Studies on the Water Absorption Properties of Short Hemp—Glass Fiber Hybrid Polypropylene Composites

Hemp fiber is one of the inexpensive and readily available bast natural fibers and hemp-fiber reinforced polymer composite products have gained considerable attraction for automotive interior products. Though extensive research has been made on the performance evaluation of these composite materials, not much data is available on the moisture absorption of the composites, which restricts their use in exterior applications. This study aims to investigate the moisture absorption of short hemp fiber and hemp-glass hybrid reinforced thermoplastic composites to study their suitability in outdoor applications. The water absorption properties and its effect on the tensile properties of hemp and hemp/glass fiber hybrid polypropylene (PP) composites prepared by an injection molding process were investigated. Effect of hybridization on the water uptake and the kinetics of moisture absorption of the hemp fiber composites were evaluated by immersing the hybrid composite samples in distilled water at different temperatures, of 40, 60 and 80°C. The composites showed a Fickian mode of diffusion; however, a deviation was observed at higher temperature and may be attributed to the microcraks developed at the interface and dissolution of the lower molecular weight substances from the natural fibers. Equilibrium moisture content (Mm) showed that water up take of 40 wt% hemp fiber reinforced PP composites was highest and incorporation of glass fiber decreased the water uptake significantly (40%). Equilibrium moisture content was found to be independent of temperature, while diffusion coefficient (D) was increased with temperature. Effect of water absorption on the tensile properties of the composites showed that there is a significant reduction in strength and stiffness. It was observed that hybridization with glass fibers did not have any significant effect on the strength properties of the aged samples. The tensile properties of the re-dried aged samples showed an increased retention of strength properties after drying; however a complete recovery of the properties has not been achieved. This indicated that water absorption is not a physical process and permanent damage occurred to the composite after aging.

High Stiffness Natural Fiber-Reinforced Hybrid Polypropylene Composites

Abstract Natural fibers are potentially a high‐performance non‐abrasive reinforcing fiber source. In this study, pulp fibers [including bleached Kraft pulp (BKP) and thermomechanical pulp (TMP)], hemp, flax, and wood flour were used for reinforcing in polypropylene (PP) composite. The results show that pulp fibers, in particular, TMP‐reinforced PP has the highest tensile strength, possibly because pulp fibers were subjected to less severe shortening during compounding, compared to hemp and flax fiber bundles. Maleic‐anhydride grafted PP (MAPP) with high maleic anhydride groups and high molecular weight was more effective in improving strength properties of PP composite as a compatiblizer. Coupled with 10% glass fiber, 40% TMP reinforced PP had a tensile strength of 70 MPa and a specific tensile strength comparable to glass fiber reinforced PP. Thermomechanical pulp was more effective in reinforcing than BKP. X‐ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM) were used to aid in the analysis. Polypropylene with high impact strength was also used in compounding to improve the low‐impact strength prevalent in natural fiber‐reinforced PP from injection molding.

Wood-polymer composites

provides a comprehensive survey of major new developments in wood-polymer composites - reviews the key aspects of manufacture, including raw materials and manufacturing technologies - discusses pro ...

Creep fatigue in engineered wood fiber and plastic compositions

Creep behaviour of unmodified and functionally modified thermoplastic-wood fibre composites was studied. For PVC, PE and PP-based composites creep is strongly dependent on the amount of load, time and temperature. A small rise in the temperature above ambient temperature increased creep significantly for PVC-woodfiber composites. Instantaneous creep resistance of woodfibre-filled PP is higher than that of PE-based composites. PP and PE-based wood composites were modified with maleic and maleimide compounds. Maleic or maleimide modification of woodfibre improved transient creep behaviour of PP-woodfibre composite but it did not show practically any effect on instantaneous creep. A mathematical model has been proposed to predict creep behaviour of PVC, PP an PE-based wood fiber composites.